1. Field of the Invention
The invention relates to a discharge lamp, especially to a discharge lamp filled with at least 0.15 mg/mm3 of mercury, argon (Ar) and halogen.
2. Description of the Prior Art
A super-high pressure mercury lamp is used as a light source for a projector. In this mercury lamp, generally, there is a pair of opposed electrodes at a distance of roughly 2 mm from one another in a silica glass arc tube which has an emission space that is filled with at least 0.15 mg/mm3 of mercury, a rare gas with argon as the main component, and halogen. This lamp is disclosed, for example, in Japanese Patent Nos. 2829339 and 2980882 (which correspond to U.S. Pat. Nos. 5,109,181 and 6,271,628) and the like. These discharge lamps are used for a liquid crystal cell device, a projector device using a DMD (digital micro mirror device), such as DLP (digital light processor) or the like, and a rear projection television.
The main purpose of adding halogen is to prevent devitrification of the arc tube. However, in this way, both the function of a so-called halogen cycle and also the action of prolonging the service life are obtained.
So that the halogen cycle works well, a suitable amount of halogen must be added. However, it is known that a suitable amount of oxygen is added, because in the case of an overly small or large amount of oxygen, the disadvantage of blackening of the inside surface of the arc tube or wear of the tungsten electrodes occurs. For example, it is described in Japanese Patent Application Publication 2004-303573 (U.S. Patent Application Publication 2004/0189208 A1) that a given amount of oxygen should be added with respect to the amount of halogen added.
Generally, the process for determining the substance contained in the emission space of the discharge lamp is a process in which spectra are measured, in which, therefore, based on the intensity of the line spectrum of a certain substance, the amount of the latter which has been added is measured. In the case of oxygen, this metering takes place based on the intensity of the line spectrum of the OH radicals.
In a discharge lamp which contains a large amount of mercury, such as a mercury lamp, especially a discharge lamp for a projector device, emission by mercury is, however, too strong, so that it is not possible to measure the line spectrum of the OH radicals. Therefore, the process for measuring a substance contained in a discharge lamp based on its spectrum for rated operation (arc discharge emission) is difficult in the case of a mercury lamp.
Furthermore, there is also a process in which the discharge lamp is subjected to a glow discharge and the substance (filler) is metered because the line spectrum of the OH radicals can be measured since the emission by mercury is not strong. This technique is described, for example, in Japanese Patent Application Publication Nos. 2002-75269 A and 2004-8204 A (the latter corresponding to U.S. Patent Application Publication 2004/0090183 A1). In this connection, OH radicals and other substances, such as argon and the like, are metered based on the respective ratio of the spectral intensity. In particular, in Japanese Patent Application Publication 2004-158204 A and corresponding U.S. Patent Application Publication 2004/0090183 A1, the ratio of the emission intensity of OH radicals to argon and the ratio of the emission intensity of hydrogen to argon are measured and the emission spectra of OH radicals and hydrogen with respect to argon are determined.
The reason for using the spectrum of OH radicals to determine the oxygen is the following:
Oxygen atoms (O) and oxygen molecules (O2) often react with other substances. Measurement of the line spectrum of oxygen atoms (O) and of oxygen molecules (O2) is difficult. In practice, OH radicals are formed by dissociation from water molecules (H2O), which have been produced for their part by a reaction of oxygen with hydrogen. The emission intensity is proportional to the number of water molecules (H2O).
However, here, a new disadvantage has arisen. The above described analysis process, based on a glow discharge, is a process in which the gas portion in the gaseous phase is measured at a relatively low temperature.
In general, in a discharge lamp, during operation, a certain amount of oxygen in the form of compounds with tungsten and bromine are present in the gaseous state and are used for the halogen cycle. When the lamp is turned off and the temperature drops, these oxygen compounds, in bound form, are converted into a solid aggregate state and are deposited on the inside wall of the arc tube and the like. Specifically, they are present as tungsten oxide (WOx) and tungsten bromoxide (WOxBry).
The oxygen present in the emission space during rated operation (arc discharge emission) as a compound of tungsten and bromine in the state of a gaseous phase contributes to the halogen cycle. However, in a glow discharge emission in the low temperature state, it is not sufficiently present in the gaseous phase, but is generally present as a compound, therefore in the solid aggregate state.
Therefore, in the process for determining the OH radicals by a conventional glow discharge, only part of the oxygen which is randomly present in the gaseous phase state is measured and oxygen in the solid aggregate state is not measured, in other words, not the oxygen which is actually to be measured. As a result, in conventional analysis processes by glow discharge emission, exact metering of the oxygen which is intended to contribute to the halogen cycle does not take place.
Of course, it can also be imagined that a process can be used in such a manner that a given amount of oxygen is reliably added in production. The reason for this is the following:
When the oxygen has been exactly added according to the design, the amount of oxygen can be fixed at a certain range even if the discharge lamp cannot be quite exactly established as a finished part.
This idea may be theoretically correct. The oxygen to be added to the discharge lamp is, however, closely connected to the production process with respect to heat treatment conditions of the material components, the atmosphere and the like. Therefore, agreement of the set point with the actual amount added is not possible.
Specifically, dissolved water or absorption water is deposited on the tungsten electrodes and the molybdenum metal foils. Furthermore, in the process of sealing the hermetically sealed portions, silica glass can vaporize and the oxygen component can penetrate into the emission space. This means that, in the production process, oxygen is present which cannot be controlled and which inevitably penetrates.
In the discharge lamp which is used for a projector device, the arc tube has an extremely small inside volume of at most roughly 100 mm3. Even for a small difference in the amount of oxygen, the function and action of the halogen cycle change greatly.
Furthermore, it can be imagined that, in the completed discharge lamp, the amount of oxygen will be measured by a destructive process. However, at the instant of destruction oxygen inevitably penetrates from the outside.
As a result, control of the amount of oxygen in the production process and a process in which the discharge lamp is destroyed after production and the oxygen is measured, cannot be implemented in practice. Therefore, the amount of oxygen actually added to the discharge lamp must be measured by a nondestructive process.
In summary this means the following:                (1) In a discharge lamp to which halogen has been added, it is necessary to add oxygen in a certain range with respect to the amount of the halogen added in order to allow the halogen cycle to proceed effectively. In JP Patent Application Publication 2004-303573 A (corresponding to U.S. Patent Application Publication 2004/0189208 A1), the range of the amount of oxygen with respect to the amount of the halogen added is shown. However, how metering is carried out is not indicated here. Nor does this reference provide any exact determination of the correctly functioning amount of oxygen in rated operation.        (2) There is a process in which the amount of the emission substance added is measured based on the emission spectra. If the discharge lamp is subjected to an arc discharge, due to the strong emission of mercury, the spectra of other substances cannot be measured. As is shown in Japanese Patent Application Publication 2002-75269 A and Japanese Patent Application Publication 2004-158204 A (corresponding to U.S. Patent Application Publication 2004/0090183 A1), there is also a metering process based on a glow discharge. Since oxygen is shifted into a solid aggregate state, the oxygen which passes into the gaseous phase during rated operation cannot be exactly measured.        